The high fixed costs associated with modern manufacturing processes provide a strong incentive for reducing downtime in industrial applications. Furthermore, today's manufactured components must be produced to more exacting tolerances than were previously required. Accordingly, manufacturers must produce larger quantities of high quality parts in order to compete in today's marketplace.
Workpiece-holding fixtures are an integral part of many fabrication processes. Work-holding fixtures keep a workpiece in place through locating and clamping during a machining operation. The term “workpiece” is to be understood to include both objects that require work and tools that perform workshop-machining work. Both positional accuracy and clamping force repeatability contribute to efficiency and quality improvement as they directly contribute to reduction in production cycle times. Thus, the ability to quickly and accurately position and clamp components into a fixture, and to quickly remove them after machining, factors greatly in the productivity of many processes.
Quick and accurate clamping is likewise required when holding a cutting tool during machining. Current popular cutting implements, such as drill bits, milling cutters, or the like, utilize round shafts that are grasped and rotated about an axis. Regardless of the cutting implement employed, it is necessary to have these cutting implements operate substantially perpendicularly to the rotating tool that is holding it. Thus, these stringent alignment requirements fall to the device to which the tool attaches. Devices commonly used to hold round-shafted workpieces in current conventional production include chucks, collets and V-blocks.
Chucks are typically cylindrically shaped devices with two, three or four moveable jaws. The common chuck is a three-jaw gear chuck, which has three jaws inclining by one-hundred and twenty degrees (120°) about the circumference. These jaws are spaced at intervals for engaging with a threading cone by means of a nut, whereby each jaw may sliding obliquely along the threads in the nut for extending or retreating. The nut is controlled by the wrench of an umbrella gear on the chuck for chucking or releasing the workpiece.
Chucks are beneficial because they allow the user to accommodate a fairly wide range of shaft sizes and, consequently, to quickly change tools between operations. However, conventional chucks are poorly suited to applications requiring precise tolerances, as the stack tolerances on the many adjustable parts of the chuck mechanism make it inherently inaccurate at centering the shaft to be rotated. When a rotating shaft is not centered, it displays what is termed as “run-out”. Run out is orbital or rotational movement at the object's end furthest from the spinning device, causing holes to be larger and/or elliptical in shape, and causing inaccuracies in milling operations.
Run out is not a problem relegated solely to cutting implements. Many operations require a spinning workpiece, for example lathe work. Once again, inaccuracies surface, as any play in the lathe chuck contributes to run out. In their fabrication, lathe chucks typically have extra assembly considerations for which a manufacturer must compensate. Many conventional lathe chucks are composed of distinct removable jaws that fit onto a spiral-shaped worm gear one jaw at a time. To operate the lathe chuck, the spiraled worm gear must accommodate the first jaw, then the worm gear must be rotated until its single tracked opening reached another jaws position on the chuck assembly. This continues until all of jaws are on the gear. As the jaws can only be attached one at a time and the worm gear constantly rotates during the assembly, the jaws that are attached first approach the center more rapidly than ones subsequently attached. To compensate for this imbalance, lathe chuck manufacturers typically insert a grinder into the gripping portion of the jaws of the fully assembled chuck and grind the jaw surfaces until each jaw surface is equidistant from a point in their center. Thus, accurate workpiece centering is directly related to grinding accuracy.
In addition to inaccuracies caused by grinding, additional inaccuracy is introduced by the necessary play between the jaw and the worm gear that drives it. As previously described, worm gears have a spiral track onto which the jaws fit. Since the jaws have a grip that must slide along this track, there must be enough space between the grip and the track so as to allow a smooth slide towards the final chuck assembly position. This space creates play, which causes the assembled chuck to display operational run out.
Collets are typically used in applications were precision is required. A collet is a ring, band, or collar that is included as one of the components in a machine-tool holder. The collet is the component of the chuck that grips and releases a workpiece. Collets hold a round shaft more completely around, but have a more limited range, for example ⅛″+/− 1/32″ or ¼″+/− 1/16″. These are generally used because common tool shafts are manufactured in specific sizes, and because greater precision may be obtained than could otherwise be realized by using conventional chucks. However, collets are not adapted for use with a variety of shaft diameters and, accordingly, must be removed and replaced whenever a different tool is needed. Replacing a collet takes a significant amount of time. Unlike chucks, the entire collet is taken from a machine during the change. After it is unscrewed, its replacement must be screwed into place, the shaft must be properly placed and then the collet is retracted into the machine. Retracting the collet presents most of the alignment problems associated with collets as during this process the machine contracts the collet's frustum shape causing it to grip the shaft. Retracting either too much or too little affects the collet's surface area gripping the bit, causing the tool to run out. Furthermore, collets are built to hold very specific sizes of shafts and even minute variations can cause run out as well.
A V-block is a conventional accessory for holding a workpiece in surface grinding work. A V-block often includes two major components: a block having a finely ground channel at a specified angle, and a clamp positioned directly above the channel. V-blocks can typically hold round, square or irregularly shaped workpieces. Unfortunately, the same versatility that allows the V-block to hold these various shapes prevents it from having direct rotational stability when attached to a spin jig. Because the work object must be placed in the center of the spin jig's rotation, the often heavy V-block must be placed to one side with the often lighter clamp on the other side. This weight disparity can cause substantial run out, especially on smaller spinning mechanisms. Additionally, the V-block must be adjusted based on the size of the workpiece. Larger workpieces will necessarily require that the V-block be positioned farther to the side and vice versa. To mitigate such a problem, a spinning mechanism need possess variable means to hold the V-block.
Therefore, there is a need for a device that can easily, quickly and effectively hold a variety of workpieces having varying widths, yet still maintain the shaft in a substantially aligned position relative to itself; and that may be attached to a spinning element to create a spinning assembly with minimal run-out.